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Error Logs from JSON Formatters: How to Interpret Them

When working with JSON data, encountering error messages from JSON formatters and validators is common. These error logs might initially seem cryptic, but they contain valuable information that can help you quickly identify and fix issues in your JSON documents. This article will help you interpret these error messages and resolve the underlying problems efficiently.

Common JSON Error Log Formats

Different JSON formatters and parsers generate error messages in various formats, but most include similar key pieces of information.

Typical Error Log Components:

Component	Example	Purpose
Error Type	SyntaxError	Categorizes the nature of the error
Error Message	Unexpected token }' in JSON at position 42	Describes what went wrong
Position/Line Number	Line 3, column 15	Pinpoints the error location
Context Snippet	"name": "value",}	Shows the problematic section of code
Expected/Found	Expected ',' or ']' but found '}'	Explains what the parser expected versus what it found

Decoding Common Error Messages

1. Unexpected Token Errors

One of the most common error types, "unexpected token" errors, occur when the parser finds a character it wasn't expecting at a particular position.

Error Example:

SyntaxError: Unexpected token '}' in JSON at position 42

Invalid JSON:

{
  "name": "John",
  "age": 30,
}  // <-- Extra comma before closing brace

Interpretation:

This error tells you that the parser found a closing brace ('}') at position 42 when it was expecting something else. In JSON, trailing commas are not allowed. The parser expected a new key-value pair after the comma but found the closing brace instead.

Fix:

{
  "name": "John",
  "age": 30
}  // Removed the trailing comma

2. Unexpected End of Input

This error occurs when the JSON string ends prematurely, often due to unclosed brackets, braces, or quotes.

Error Example:

SyntaxError: Unexpected end of JSON input

Invalid JSON:

{
  "user": {
    "name": "Alice",
    "email": "alice@example.com"
  // Missing closing brace

Interpretation:

The parser reached the end of the input but was still expecting more characters to complete the JSON structure. In this case, the document is missing the closing brace for the outer object.

Fix:

{
  "user": {
    "name": "Alice",
    "email": "alice@example.com"
  }
}  // Added two closing braces

3. Expected Property Name or ‘}’

This error often occurs when there's an issue with a property name or a missing closing brace.

Error Example:

SyntaxError: Expected property name or '}' at line 4, column 3

Invalid JSON:

{
  "name": "John",
  "age": 30,
  , // <-- Extra comma
  "city": "New York"
}

Interpretation:

The parser found a comma at line 4, column 3, but what should follow a comma is either a property name (for an object) or a closing brace (for an empty object). Instead, it found another comma, which is not valid in JSON syntax.

Fix:

{
  "name": "John",
  "age": 30,
  "city": "New York"
}  // Removed the extra comma

Pro Tip:

When you see position numbers in error messages (e.g., "at position 42"), remember that these are zero-indexed character counts from the start of the document. Line and column numbers are typically more intuitive for locating errors in larger JSON files.

4. Unterminated String

This error occurs when a string is not properly closed with a matching quotation mark.

Error Example:

SyntaxError: Unterminated string at line 3, column 24

Invalid JSON:

{
  "name": "John",
  "message": "Hello world  // <-- Missing closing quote
}

Interpretation:

The parser reached the end of the line (or file) while processing a string but never found the closing quotation mark.

Fix:

{
  "name": "John",
  "message": "Hello world"  // Added closing quote
}

5. Unescaped Control Characters

JSON has specific rules for including control characters, which must be properly escaped.

Error Example:

SyntaxError: Bad control character in string literal at line 3, column 21

Invalid JSON:

{
  "name": "John",
  "note": "Meeting at
  2pm tomorrow"  // <-- Literal newline in string
}

Interpretation:

In JSON, control characters like newlines must be escaped with a backslash followed by a specific character code. Here, a literal newline appears in the string, which is not allowed.

Fix:

{
  "name": "John",
  "note": "Meeting at\n  2pm tomorrow"  // Used \n escape sequence
}

Error Logs in Different Environments

1. Browser Console

Browser development tools provide detailed error information when parsing JSON.

Chrome Developer Console Example:

Uncaught SyntaxError: Unexpected token , in JSON at position 39
    at JSON.parse (<anonymous>)
    at parseJSON (script.js:15:23)
    at processData (script.js:24:12)
    at HTMLButtonElement.onClick (script.js:32:5)

Browser errors often include a stack trace showing where the error occurred in your code. This helps you trace back to which function or event handler triggered the JSON parsing.

2. Node.js

Node.js provides detailed error information similar to browsers but with some differences in formatting.

Node.js Error Example:

SyntaxError: Unexpected token } in JSON at position 42
    at JSON.parse (<anonymous>)
    at Object.<anonymous> (/path/to/script.js:5:19)
    at Module._compile (internal/modules/cjs/loader.js:1085:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)
    at Function.Module._load (internal/modules/cjs/loader.js:790:12)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)
    at internal/main/run_main_module.js:17:47

3. JSON Validation Tools

Dedicated JSON validation tools often provide more user-friendly error messages with visual indicators.

JSON Validator Tool Example:

Error: Parse error on line 4:
{
  "name": "John",
  "age": 30,
---------^
Expecting 'STRING', 'NUMBER', 'NULL', 'TRUE', 'FALSE', '{', '[', got ','

Many JSON validators provide visual indicators like the caret (^) pointing to the exact position of the error, making it easier to identify the problem.

Advanced Debugging Techniques

1. Using Diff Views

When working with large JSON files, using a diff view to compare valid and invalid versions can help identify subtle issues.

Diff View Technique:

Save a backup of your original JSON
Make incremental changes and validate after each change
When you find a working version, compare it with the original using a diff tool
The highlighted differences will pinpoint the exact issues

2. Binary Search Method

For very large JSON files, use a binary search approach to locate errors.

Binary Search Approach:

Split the JSON in half
Add closing brackets/braces to make each half valid JSON syntax (if needed)
Validate each half separately
Continue dividing the invalid section until you isolate the error

This approach can quickly narrow down the location of an error in large JSON files, making it easier to fix.

3. Incremental Building

Instead of fixing a broken JSON document, sometimes it's more efficient to rebuild it from scratch.

Incremental Building Process:

Start with a minimal valid JSON structure (e.g., {} or [])
Add one section at a time, validating after each addition
If an error occurs, you'll know exactly which section caused it
Continue until you've rebuilt the entire document

Common Error Patterns and Solutions

Quick Reference: Common Errors and Fixes

Error Message Pattern	Likely Cause	Solution
Unexpected token }	Trailing comma	Remove the trailing comma before closing bracket/brace
Expected property name or }	Malformed property name or extra comma	Ensure property names are in quotes; check for extra commas
Unexpected end of input	Missing closing bracket/brace/quote	Add the missing closing character
Unterminated string	Missing closing quote	Add the missing quote; check for unescaped quotes inside strings
Bad control character	Unescaped newline or other control character	Use escape sequences (\\n, \\t, etc.) instead of literal control characters
Duplicate key	The same property name appears multiple times in an object	Rename or remove the duplicate keys
Expected ':' after property name	Missing colon between property name and value	Add the missing colon

Best Practices for Preventing JSON Errors

Use a linter or validator during development to catch errors early
Format your JSON properly with consistent indentation to make it more readable
Generate JSON programmatically instead of writing it by hand when possible
Use language-specific JSON libraries (like JSON.stringify() in JavaScript) that handle proper escaping
Implement validation before parsing in production applications to provide better error messages
Comment your complex JSON structures (in development versions only, remove for production)

JSON Validation Code Example:

// JavaScript example of user-friendly JSON validation
function parseJSON(jsonString) {
  try {
    return {
      data: JSON.parse(jsonString),
      error: null
    };
  } catch (error) {
    let errorMessage = 'Invalid JSON';
    let errorLine = null;
    let errorColumn = null;
    
    // Check if we can extract line/column information
    if (error instanceof SyntaxError) {
      // Example message: "Unexpected token } in JSON at position 42"
      const posMatch = error.message.match(/at position (\d+)/);
      
      if (posMatch) {
        const pos = Number(posMatch[1]);
        
        // Calculate line and column number from character position
        const upToError = jsonString.substring(0, pos);
        const lines = upToError.split('\n');
        errorLine = lines.length;
        errorColumn = lines[lines.length - 1].length + 1;
        
        // Extract context around the error for better readability
        const errorContext = jsonString.substring(
          Math.max(0, pos - 20),
          Math.min(jsonString.length, pos + 20)
        );
        
        errorMessage = `${error.message}
Line: ${errorLine}, Column: ${errorColumn}
Context: "...${errorContext}..."`;
      } else {
        errorMessage = error.message;
      }
    }
    
    return {
      data: null,
      error: errorMessage
    };
  }
}

Conclusion

Understanding and interpreting error logs from JSON formatters is an essential skill for anyone working with JSON data. While the error messages might initially seem cryptic, they provide valuable information about the nature and location of syntax issues in your JSON documents.

By learning to read these error messages and applying the appropriate fixes, you can quickly resolve JSON syntax problems and ensure your data is correctly formatted. As you gain experience, you'll find that most JSON errors fall into predictable patterns with straightforward solutions.

Remember that prevention is often easier than cure—using tools like linters, formatters, and programmatic JSON generation can help you avoid many common JSON syntax errors before they occur.

Need help with your JSON?

Try our JSON Formatter tool to automatically identify and fix syntax errors in your JSON. JSON Formatter tool

Need help with your JSON?

Error Logs from JSON Formatters: How to Interpret Them

Common JSON Error Log Formats

Typical Error Log Components:

Decoding Common Error Messages

1. Unexpected Token Errors

Error Example:

Invalid JSON:

Interpretation:

Fix:

2. Unexpected End of Input

Error Example:

Invalid JSON:

Interpretation:

Fix:

3. Expected Property Name or ‘&rbrace;’

Error Example:

Invalid JSON:

Interpretation:

Fix:

Pro Tip:

4. Unterminated String

Error Example:

Invalid JSON:

Interpretation:

Fix:

5. Unescaped Control Characters

Error Example:

Invalid JSON:

Interpretation:

Fix:

Error Logs in Different Environments

1. Browser Console

Chrome Developer Console Example:

2. Node.js

Node.js Error Example:

3. JSON Validation Tools

JSON Validator Tool Example:

Advanced Debugging Techniques

1. Using Diff Views

Diff View Technique:

2. Binary Search Method

Binary Search Approach:

3. Incremental Building

Incremental Building Process:

Common Error Patterns and Solutions

Quick Reference: Common Errors and Fixes

Best Practices for Preventing JSON Errors

JSON Validation Code Example:

Conclusion

Need help with your JSON?

3. Expected Property Name or ‘}’